The present invention relates to a thermoformed panel obtained from a sheet of thermoformable plastics material, and to a process for its preparation.
Of particular interest within the scope of the invention are thermoformed panels of the type conventionally utilised in the motor vehicle industry such as, for example, structural or cladding panels, wheel arch panels, head linings and the like; however, it is intended that the invention is not to be limited to specific applications.
The thermoforming procedure for thermoformable plastics material is well known in the art and is advantageous from the economic point of view for the production of shaped panels. One limitation of this process is generally that the thermoformed articles thus obtained have opposite faces shaped in such a way as to be complementary to one another. If it is desirable to produce panels which have reinforcing ribs or ridges, for example to improve the mechanical strength against flexing, or to obtain on the surface of the panel projections of an aesthetic or useful character to shape the panel according to predetermined shapes, the conventional thermoforming process implies the deformation of both faces of the panel. In general, the presence of a rib projecting from one face of the panel causes the presence, on the other face, of a corresponding concavity and this constitutes a limitation in relation to the shapes which can be obtained by thermoforming.
Particularly in the case of panels of large dimensions the formation of reinforcing ribs is often an essential requirement for the purpose of achieving an adequate structural rigidity and, as mentioned, this results in constraints in relation to the aesthetic aspect of one of the two faces of the panel.
The present invention has the object of overcoming the said limitations. To this end a first object of the invention is a process for the production of a panel comprising a sheet of thermoformable plastics material having separate regions with a greater thickness than the basic thickness of the sheet, characterised in that it comprises:
extrusion of the plastics material sheet with the addition to the said material of an expanding agent, the extrusion being conducted in conditions such as to prevent expansion of the material or to cause only a partial expansion of the material;
reheating of the thus-obtained sheet to a temperature such as to cause the post-expansion of the material; and
thermoforming of the sheet in a thermoforming cavity of shape complementary to that of the desired panel so that the thermoformed panel produced has, at least in the regions having a greater thickness than the basic thickness, an expanded cellular structure where the material has a lower density than the density of the material in the regions having the basic thickness.
It is intended that, taking into account the addition of an expanding agent to the plastics material, which in the extrusion is distributed in a substantially uniform manner within the mass of the material, the panel produced will generally have, even in the regions of minimum thickness, a foamed cellular structure where, however, the material has a greater density than the density of the material in the zones of greater thickness.
The invention also includes a thermoformed panel as defined in the following claims, obtainable with the above-described procedure.